1. Field of the Invention
The present invention relates to a food material containing docosahexaenoic acid (C.sub.21 H.sub.31 CO.sub.2 H), and more particularly to a food material that does not smell offensively of fish oil or the like. Also, the present invention relates to a food using the foregoing food material.
The docosahexaenoic acid is a highly-unsaturated fatty acid having, according to reports made recently, a variety of beneficial physiological functions, such as lowering cholesterol, preventing blood coagulation and improving learning capability.
2. Description of the Related Art
It has been known that the docosahexaenoic acid (hereinafter abbreviated to "DHA"), which is highly-unsaturated fatty acid reported to have a variety of beneficial physiological functions, is contained in fish oil. This fact has resulted in health foods, which use the fish oil as the raw material, being sold. However, health foods using the fish oil as the raw material encounter a difficulty in removing the smell of the fish. To alleviate this problem, the fish oil is usually formed into pills coated with gelatin or the like. However, under the foregoing circumstance, it is difficult to eat the food of the foregoing type when it is mixed with a usual food.
Since DHA can be oxidized very easily, a food which is manufactured by simply adding DHA or fish oil containing DHA cannot be cooked with heat in air. Hence, there arises a necessity of adding an antioxidation agent, or of using a means for preventing the contact with air.
Recently, a discovery of eye socket lipid textures of various fish containing highly-concentrated DHA and progress in technology of separating and refining the fatty acid found in the textures have enabled the DHA to be separated and refined. However, the necessity of using very expensive reagents to separate and refine the DHA increases the cost of the food. Moreover, there have been problems associated with a peculiar smell caused from the fish oil, an unstable supply of the raw material, and so forth.
In order to overcome the foregoing problems, a method has been discovered which causes microorganisms or the like to selectively produce DHA. For example, Pratima Bajpai et al., in their research, have reported that Thraustochytrium aureum belonging to lower fungi, produces DHA (APPL. MICROBIOL. BIOTECHNOL., 35,706 (1991)). However, because the aforementioned fungus is a photosynthetic fungus, it is necessary to use light to culture it, thus requiring a special and large-sized culturing apparatus. What is worse, other highly-unsaturated fatty acids, such as arachidonic acid and eicosapentaenoic acid, which have similar physical properties, are produced by 10 to 20 wt %, thus requiring an advanced separating and refining apparatus.
R. J. Henderson reported that the highly-unsaturated fatty acid contained in a lipid produced by Crypthecodinium cohnii, which is a marine micro algae performing no photosynthesis, is only DHA, and its content with respect to the overall weight of the fatty acid is 9 wt % (PHYTOCHEMISTRY, 27 (6) 1679 (1988)). However, there are problems in that the required static culturing method cannot be adapted to mass-quantity culturing, and that the content of the DHA is unsatisfactorily low. Further, International Publication Nos. WO91/11918 and W092/12711 disclose a method of using a unicellular edible oil, which is extracted from C. cohnii and contains DHA, for mixed infant milk. In the method, when using DHA, a preventive measure for oxidation is also necessary, since DHA is extracted from marine algae.